The present invention relates to a pressure-relief valve device, particularly for use with hydraulic props. Various hydraulically operated units of mine equipment need to be protected against excessive pressure. This is especially the case with hydraulic props which support the roof of a mine working. Normally such props utilize pressure fluid--usually oil-water emulsions--at a pressure in the range 200-600 bars. Under adverse conditions, the pressure in the working chamber of a hydraulic prop may rise suddenly. A small compact pressure relief valve device connected to the working chamber can relieve excess pressure and protect the prop should the pressure rise. Known valve devices operate entirely automatically and open and close according to the prevailing pressure. An example of a pressure-relief valve device is described in German Gebrauchsmuster No. 1945206.
Exacting demands are made on the valve devices for relief of pressure since they are expected to perform reliably over long periods of time and they are expected to remain highly sensitive to pressure variation despite the harsh conditions encountered in the mine working environment. Conventional pressure-relief valve devices adopted in mine workings for the relief of hydraulic props perform quite adequately but are only designed for comparatively small flows of hydraulic pressure fluid. This is not generally a disadvantage, however, since in the event of a pressure rise and an overload only a small quantity of pressure fluid needs to be removed to restore the pressure to a safe level. Nevertheless, where the hydraulic props are exposed to a normal setting load which is in the same order of magnitude as the relief setting at which the pressure-relief valve devices are adjusted to respond; problems can arise where the devices can only pass pressure fluid at a relatively low flow rate. Indeed in this case during the actual prop setting operation the relief devices can respond and would then need to convey the large quantities of fluid passing through the pressure feed line. There is thus a need for a pressure-relief valve device which will be able to transfer larger quantities of fluid--in the order of 50 liters per minute-- than hitherto while still preserving the efficiency reliability and durability which is needed.
A general object of the present invention is to provide an improved pressure-relief valve device.